Electronic assemblies commonly employ one or more printed circuit boards in their construction. Such circuit boards provide mounting points for electronic components and/or for sockets that allow other circuit boards, cables, or device packages to connect to the circuit board. The circuit board provides conductive traces, and possibly planar conductive regions, patterned on conductive layers sandwiched between insulating dielectric layers. A typical circuit board may contain anywhere from a few conductive layers to upwards of thirty such layers for complex systems. Conductive traces route signals (and possibly power) from one point on the circuit board to another point on the circuit board. Planar conductive regions are employed for power distribution. Planar conductive regions also serve as reference planes, which when coupled through a dielectric layer to one of the conductive traces or a differential pair of such traces, form stripline transmission lines of specific impedance. Plated through-holes (PTHs) in the circuit board can form mounting points for press-fit devices, allow for signal insertion/extraction to the internal board layers, and can also serve as layer-swapping vias that transfer a signal from a trace on one conductive layer to another trace on another conductive layer.
FIGS. 1A and 1B illustrate a circuit board portion 100 containing some common structures found within the internal layers of a printed circuit board. The cross-section of FIG. 1B shows three traces 110, 120, 130 on a common conductive layer, four dielectric layers 134, 136, 140, and 142, and conductive plane layers 160 and 170. It is understood that other layers above and below these layers can exist in a complete circuit board. The plan view of FIG. 1A shows only the traces 110, 120, 130, the dielectric layer 140, and the conductive plane layer 160, which in circuit board portion 100 is coextensive with dielectric layer 140 except for the clearances 162, 164 indicated by hidden lines, where the plane layer 160 is removed so that it does not short to plated through-holes co-located with pads 112, 122, 114, and 124.
Traces 110 and 120 form a differential trace pair. The pair receives a signal pair at through-holes connected to pads 112 and 122, and propagates the signal to another pair of through-holes co-located with pads 114 and 124. The signals are differential with respect to the reference planes 160 and 170—at any point along the traces, the voltage on trace 110 will have approximately the opposite polarity and the same magnitude as the voltage on trace 120, as referenced to the voltage on planes 160 and 170. The impedance of the configuration is determined by the differential coupling of the electromagnetic (EM) fields between the two traces and the single-ended coupling of each trace to planes 160 and 170. The spacing between the traces, spacing between the trace layer and the plane layers, and trace size are adjusted to achieve a desired characteristic impedance. Such trace pairs are commonly used to transmit high-speed signals (digital symbol rates greater than 1 billion symbols/second) between a source component and a receiver component.
Trace 130 is a single-ended trace. Such a trace is routed at a distance from the differential pair and other conductors (not shown) such that its characteristic impedance is dominated by its single-ended coupling to reference planes 160 and 170. Such traces are generally used for lower-speed signals, where the performance of a differential pair is not required, to reduce space and componentry requirements.
Another feature shown in FIG. 1A is thieving, which comprises a pattern of dummy lands (see numbered dummy land 150) patterned on the same conductive layer as traces 110, 120, and 130. These lands are placed in large unused regions of a conductive layer to help preserve the planarity of the circuit board during construction.